1. Field of the Invention
This invention relates generally to modules used in computers, such as single in-line memory modules (SIMMs) and the like. More specifically, the present invention relates to a SIMM housing including a smaller printed circuit board to form a standard outline module.
2. State of the Art
An integrated circuit (IC) typically includes a semiconductor die (die) electrically attached to a lead frame providing physical support for the die and connecting the die to external circuitry, such as a printed circuit board. In such an arrangement, the lead frame and die are connected by forming wire bonds between the lead fingers of the lead frame and the bond pads located on a surface of the die. The die and lead frame are then typically encapsulated within a plastic package, although ceramic and metal packages may also be used, depending on the operating environment and the packaging requirements of the die.
As the demand for memory, in particular random access memory (RAM), surpassed the memory capability of a single die, multi-chip modules (MCMs) were developed having a number of memory devices attached to a single substrate, such as a printed circuit board. A single in-line memory module (SIMM) is a multi-chip memory module having multiples of the same basic die, where the semiconductor memory chips are aligned in a row and interconnected to a printed circuit board to, in effect, create a single device with the memory capacity of the combined memory chips. Internal circuitry of the printed circuit board connects each chip to terminals attached along one edge of the printed circuit board. The terminals are configured for attachment to an edge-type connector, such as a SIMM socket as is known in the art. As the demand for additional memory on a single device has increased, other devices, such as a dual in-line memory module (DIMM), have also been developed. Such devices, while providing the desired memory capability on a single printed circuit board, are relatively expensive to manufacture, requiring manufacture and/or purchase of relatively expensive components.
As an increase in supply of SIMMs in the marketplace has dropped the price of RAM, an even greater need to decrease the cost of manufacturing SIMMs exists. Because of advances in chip manufacturing technology and efficiency, manufacturers can produce thousands of chips per day. With such high volume production rates, small improvements in efficiency and/or cost reduction per chip can make substantial differences in the daily cost of manufacturing. While manufacturers have continued to streamline processes to improve efficiency and decrease production costs, the amount of materials used to form many standard components has remained relatively constant.
One of the components of a SIMM is a printed circuit board, typically comprised of a fire-retardant, epoxy resin/glass cloth laminate, commonly referred to as FR-4. The epoxy resin used in the formation of the FR-4 board is typically diglycidyl ether of 4,4'-bis(hydroxyphenyl)methane, or other low-molecular weight polymers of it. Fire retardancy is imparted by including enough tetrabromobisphenol-A to provide 15% to 20% bromine content. Sometimes, about 10% of higher functionality epoxy is added to raise the glass transition point and improve chemical resistance of the resin by increasing cross-link density.
Other materials used in the manufacture of FR-4 boards include curing agents such as dicyandiamide (DICY) and catalysts such as tertiary amines. DICY has low solubility in common organic solvents, so that the full stoichiometric capability for linking the epoxy-bond network is not used. The molding flow is suited for multi-layer laminating needed for printed circuit boards, and the cured physical properties are good with strain to fracture ratio of about 0.04, thereby providing enough resiliency for good metal or glass bond interfaces.
While FR-4 is the preferred material for the SIMM board, it is a relatively expensive part of the SIMM and can account for a substantial portion of the cost of the entire SIMM. Other high-performance boards based on such high glass-transition-temperature materials, such as polyamide-glass or cyanate-glass, may also be used, but are substantially more expensive than FR-4. In addition to the cost of materials, the care necessary to ensure an aesthetically pleasing component requires special attention to be paid to the quality of workmanship. This additional care may require more detailed or additional inspection of each component and slow the rate of production.
It is known in the art to provide containers for "plug-in" peripheral cards in the form of printed circuit boards contained within an exterior package. These devices are termed "PCMCIA style peripheral devices" (PCMCIA being the acronym for Personal Computer Memory Card International Association) and are designed to be plugged-in and removed by the user. Hence, the cards are typically housed within a metal case that protects the memory devices contained therein from being physically contacted and/or damaged by the user or environment. Such a device is illustrated in U.S. Pat. No. 5,397,857 to Farquhar et al.
SIMMs, on the other hand, are not typically housed and, as previously described, are simply printed circuit boards having a row of memory chips attached thereto and a row of connectors along one edge. While typically being removably attached to a computer card, SIMMs tend to remain untouched once installed in a computer unless the amount of memory is changed by adding to or replacing one or more SIMMs.
An exception to a conventional SIMM configuration is taught in U.S. Pat. No. 5,109,318 to Funari et al. As described in U.S. Pat. No. 5,109,318, a housing for a conventional SIMM serves as a heat sink. Such a heat sink housing would add to the cost of the finished SIMM.
Therefore, a need exists for the fabrication of less costly SIMMs and the like by reducing the quantity of material used to make the SIMM board while allowing the SIMM to be mounted to an industry-standard SIMM socket. In addition, the ability to cover the SIMM and any aesthetic anomalies thereon has the added benefit of speeding the production rate of SIMMs.